The invention relates generally to radio frequency energy launchers and, more particularly, to such devices for coupling a coaxial transmission line to a microstrip transmission line.
As is known in the art one technique used to connect a coaxial transmission line to a microstrip transmission line is to provide the outer conductor of the coaxial transmission line with a flange vertically secured to an end of the microstrip transmission line and electrically connected to a relatively thick ground plane conductor of the microstrip transmission line. The inner conductor of the coaxial transmission line has its end directly connected to the strip conductor of the microstrip transmission line. Thus, as shown in FIG. 1, the coaxial transmission line 18 includes an outer conductor 20 integrally formed with the flange 22 and an inner conductor 24 spaced from, and coaxial with, the outer conductor 20, as shown. Microstrip transmission line 10 includes a dielectric 14 separating a strip conductor 12 and a ground plane conductor 16, as shown. With such structure, the cantilever type attachment of the flange 22 to the microstrip ground plane 16 necessitates the use of a substantially thicker ground plane conductor for the microstrip transmission line. Further, the end of the inner conductor 24 of the coaxial line 18 has a tendency to lift vertically from engagement with the microstrip strip conductor 12 when the flange 22 is tightened, by screw means 26, to the ground plane conductor 16. Other mechanical problems arise due to the fact that the inner conductor 24 is generally so fragile that it may break when an attempt is made to provide a tight mechanical contact with the strip conductor 12 of the microstrip transmission line.
A major electrical problem of such arrangement is attributed to electromagnetic field discontinuities at the interface between the microstrip transmission line and the coaxial transmission line.
Another prior art structure is described in U.S. Pat. No. 3,622,915 issued Nov. 23, 1971 to Davo comprising an electrical coupler having a horizontal inner and a horizontal outer conductive member which have a cross section at one end identical to that of a coaxial transmission line and a cross section at the other end including a pair of spaced straight lines for connection to spaced conductive strips of microstrip transmission line. A ramplike conductive member disposed within the coupler outer conductive member has a smaller end terminating at the coaxial transmission line end and a larger end terminating adjacent to the microstrip transmission line. The horizontal inner conductive member of the coupler terminates in a springlike member for attachment to the thin strip conductor of the microstrip transmission line. The structure is relatively complicated to fabricate and there is an absence of positive mechanical contact means to interconnect to the thin microstrip conductor without soldering the interconnected components which is more difficult to achieve, particularly, at higher frequencies, such as K.sub.u band.